Everything you see around you, from your phone to the stars in the sky, is made of atoms. For a long time, we thought atoms were the smallest things possible. But they're actually made of even smaller particles.

At the center of every atom is a dense core called the nucleus. The nucleus contains two types of particles: positively charged protons and neutral neutrons. Whizzing around the nucleus are tiny, negatively charged electrons.

The number of protons determines what element an atom is. For example, an atom with one proton is always hydrogen. An atom with six protons is always carbon. The number of neutrons can vary, creating different versions of an element called isotopes, but the proton count is what defines the element's identity.

If you remember anything about magnets, you know that like charges repel each other. A nucleus is packed with positively charged protons, so they should be pushing each other apart with incredible force. Why doesn't the nucleus just fly apart?

It's held together by something called the strong nuclear force. This is the most powerful of the four fundamental forces of nature. It's about 100 times stronger than the electromagnetic force that causes protons to repel each other.

Think of it like super glue. It creates an incredibly strong bond between protons and neutrons, but only when they're touching. If you pull them even slightly apart, the force vanishes. This delicate balance between the repulsive electric force and the attractive strong force is the key to everything in nuclear energy.

So, where does the massive energy in a nuclear reaction come from? The answer lies in a famous equation from Albert Einstein.

This equation reveals a fundamental secret of the universe: mass and energy are two sides of the same coin. They can be converted into one another.

Here's what the letters mean:

• E is energy.
• m is mass.
• c is the speed of light, a very, very large number (about 300,000,000 meters per second).

Because the speed of light ($c$) is squared, converting even a tiny amount of mass releases a tremendous amount of energy. When a nucleus is split or fused in a nuclear reaction, the total mass of the particles that come out is slightly less than the mass of the particles that went in. This tiny bit of 'missing' mass hasn't disappeared. It has been converted into a huge burst of energy, according to $E = mc^2$.

Now that we understand the basic building blocks, we can explore the processes that unlock this incredible energy.

Let's check your understanding of these core concepts.

With these fundamentals in place, we're ready to look at how we can harness the power locked inside the atom's nucleus.